Trench sidewall protection for selective epitaxial semiconductor material formation

ABSTRACT

A method of forming a semiconductor device includes forming an insulator layer over a substrate; opening a trench in the insulator layer so as to expose one or more semiconductor structures formed on the substrate; forming a protective layer on sidewalls of the trench; subjecting the substrate to a precleaning operation in preparation for epitaxial semiconductor formation, wherein the protective layer prevents expansion of the sidewalls of the trench as a result of the precleaning operation; and forming epitaxial semiconductor material within the trench and over the exposed one or more semiconductor structures.

BACKGROUND

The present invention relates generally to semiconductor devicemanufacturing techniques and, more particularly, to trench sidewallprotection for selective epitaxial semiconductor material formation.

In the manufacture of integrated circuit devices such as field effecttransistors (FETs) it is sometimes desirable to selectively addsemiconductor material to an existing, preformed semiconductor structureprior continuing with subsequent processing steps. For example, inplanar transistor configurations, a raised source/drain region is asemiconductor region that is formed on a pre-existing semiconductorregion of an FET and functions as a part of the source/drain region ofthe field effect transistor. A raised source/drain region can be formedon a pre-existing source/drain region by a selective deposition process,which selectively deposits additional semiconductor material on exposedsemiconductor surfaces without depositing any semiconductor material ondielectric surfaces.

In another example, non-planar FETs incorporate various verticaltransistor structures, and typically include two or more gate structuresformed in parallel. One such semiconductor structure is known as thefinFET, which takes its name from the multiple thin silicon “fins” thatare used to form the respective gate channels, and which are typicallyon the order of tens of nanometers in width. Due to the advantageousfeature of full depletion in a finFET, the increased number of sides onwhich the gate electrode controls the channel of the finFET enhances thecontrollability of the channel in a finFET compared to a planar MOSFET.The improved control of the channel allows smaller device dimensionswith less short channel effects as well as larger electrical currentthat can be switched at high speeds. A finFET device generally hasfaster switching times, equivalent or higher current density, and muchimproved short channel control than planar CMOS technology utilizingsimilar critical dimensions.

In certain instances, it is desirable to selectively merge selectedregions of predefined fin structures, such as through selectiveepitaxial growth of a semiconductor material.

Regardless of whether a device is a planar transistor or a finFET, it isdesirable to be able to precisely control where such additional regionsof semiconductor materials are formed.

SUMMARY

In an exemplary embodiment, a method of forming a semiconductor deviceincludes forming an insulator layer over a substrate; opening a trenchin the insulator layer so as to expose one or more semiconductorstructures formed on the substrate; forming a protective layer onsidewalls of the trench; subjecting the substrate to a precleaningoperation in preparation for epitaxial semiconductor formation, whereinthe protective layer prevents expansion of the sidewalls of the trenchas a result of the precleaning operation; and forming epitaxialsemiconductor material within the trench and over the exposed one ormore semiconductor structures.

In another embodiment, a method of forming a semiconductor deviceincludes forming a plurality of semiconductor fins over a substrate;forming an insulator layer over the semiconductor fins and thesubstrate; opening a trench in the insulator layer so as to expose aselected region of the plurality of semiconductor fins; forming aprotective layer on sidewalls of the trench; subjecting the substrate toa precleaning operation in preparation for epitaxial semiconductorformation, wherein the protective layer prevents expansion of thesidewalls of the trench as a result of the precleaning operation; andforming epitaxial semiconductor material within the trench and over theexposed plurality of semiconductor fins so as to merge the semiconductorfins.

In another embodiment, a semiconductor device includes a plurality ofsemiconductor structures formed on a substrate; an insulator layerformed over a substrate; a trench formed in the insulator layer, thetrench configured to expose a portion of the plurality of semiconductorstructures; a protective layer formed on sidewalls of the trench, theprotective layer comprising a material that prevents expansion of thesidewalls of the trench as a result of a precleaning operation; and anepitaxial semiconductor material formed within the trench and over theexposed one or more semiconductor structures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1 through 9 are a series of cross sectional views illustrating amethod of protecting patterned trenches in a dielectric layer inpreparation for additional semiconductor material formation, in which:

FIG. 1 illustrates a starting semiconductor-on-insulator (SOI) substrateincluding a bulk layer, a buried insulator (BOX) layer and a thin SOIlayer, with the SOI layer patterned into fin structures;

FIG. 2 illustrates the formation of a dielectric layer over thestructure of FIG. 1;

FIG. 3 illustrates the formation of a trench in a region of thedielectric layer of FIG. 2;

FIG. 4 illustrates the formation of a protective guard layer oversurfaces of the dielectric layer, including the trench, exposed finstructures, and exposed BOX layer;

FIG. 5 illustrates a directional etch of the protective guard layer to apoint at which horizontal portions of the guard layer are removed;

FIG. 6 illustrates a continuation of the directional etch such thatvertical portions of the guard layer on the exposed fin structures areremoved, leaving vertical portions only on lower sidewall portions ofthe trench, and thereby defining a trench guard ring;

FIG. 7 illustrates a precleaning operation of the structure of FIG. 6,resulting in erosion of upper sidewall portions of the trench where theguard layer was removed;

FIG. 8 illustrates a semiconductor epitaxial growth process resulting inmerging of the exposed fin structures;

FIG. 9 illustrates silicide layer formation over the exposedsemiconductor material in the trench; and

FIG. 10 is a top view of the structure depicted in FIG. 9.

DETAILED DESCRIPTION

With respect to selective formation of additional semiconductor materialover existing, preformed semiconductor structures as described above,one approach is to selectively pattern trenches or openings in asubstrate level dielectric layer that covers the preformed semiconductorstructures. Once the trenches are opened, an additive semiconductorformation process is performed (e.g., epitaxial growth) so that theepitaxial semiconductor material is formed only on exposed semiconductorregions as exposed by the trenches in the dielectric layer.

However, one problem with this approach is the result of a hydrofluoricacid (HF) precleaning operation used in conjunction with a dielectriclayer such as SiO₂. The use of such a cleaning solution may undesirablyattack the patterned dielectric layer and enlarge the area of theopenings beyond a desired tolerance. This in turn may lead to theformation (growth) of additional semiconductor material in regions thatare not desired.

Accordingly, disclosed herein is a protection scheme for preserving theintegrity of patterned trenches in a dielectric layer in preparation foradditional semiconductor material formation. As described in furtherdetail below, the present embodiments address this issue by introducingan etch resistant guard material that preserves trench sidewallintegrity during a precleaning operation such as an HF etch. The guardmaterial prevents excessive erosion of the trench sidewalls so as toprevent undesired semiconductor material formation (e.g., fin mergingwhere not desired). As will be appreciated, the embodiments describedherein are applicable to both planar and finFET device structures.

Referring generally now to FIGS. 1 through 9, there are shown a seriesof cross sectional views illustrating a method of protecting patternedtrenches in a dielectric layer in preparation for additionalsemiconductor material formation, in accordance with an exemplaryembodiment.

As shown in FIG. 1, a substrate 100 includes a bulk semiconductor layer102, a buried insulator (e.g., oxide) (BOX) layer 104 formed on the bulksemiconductor layer 102, and an active SOI layer formed on the BOX layer104. Here, the illustrated portions of the SOI layer are shown as havingbeen patterned into patterned into fin structures 106, as known in theart of finFET processing. The semiconductor substrate material of thebulk layer 102 and/or the material SOI layer 206 may be selected from,but is not limited to, silicon, germanium, silicon-germanium alloy,silicon carbon alloy, silicon-germanium-carbon alloy, gallium arsenide,indium arsenide, indium phosphide, III-V compound semiconductormaterials, II-VI compound semiconductor materials, organic semiconductormaterials, and other compound semiconductor materials.

As further shown in FIG. 1, fin structures 106 of a first group 108 areto be subsequently merged, while other fin structures 106 of a secondgroup 110 are to remain intact. The second group 110 of the finstructures 106 is shown for illustrative purposes. Although notspecifically illustrated in the cross sectional view of FIG. 1, theremay be one or more gate stack structures formed over the semiconductorfin structures.

In FIG. 2, an interlevel dielectric (ILD) layer 112, such as silicondioxide (SiO₂) for example, is formed over the structure of FIG. 1. Moregenerally, the ILD layer 112 can include a dielectric material such asundoped silicon oxide, doped silicon oxide, silicon nitride, porous ornon-porous organosilicate glass, porous or non-porous nitrogen-dopedorganosilicate glass, or a combination thereof. The ILD layer 112 can bedeposited, for example, by chemical vapor deposition or spin-coating. Ifthe top layer of the ILD layer 112 is not self-planarizing, then the topsurface of the ILD layer 112 may be planarized, for example, by chemicalmechanical planarization (CMP).

FIG. 3 illustrates the formation of a trench 114 in a region of the ILDlayer 112 of FIG. 2, corresponding to the first group of fins 106 to bemerged. In this regard, a photoresist layer (not shown) is applied overthe ILD layer 112, and is lithographically patterned to form variousopenings therein. The various openings overlie a portion of the devicewhere it is desired to merge at least some the plurality of fins 106.The pattern in the photoresist layer is then transferred through the ILDlayer 112 so as to form the trench 114 shown in FIG. 3. An anisotropicetch having a chemistry that removes the dielectric material(s) of theILD layer 112 selective to the semiconductor fin material may beemployed.

Once the trench 114 is defined, FIG. 4 illustrates the formation of aprotective guard layer 116 over surfaces of the ILD layer 112, theexposed fin structures 106, and exposed BOX layer 104. The protectiveguard layer 116 may be any suitable layer(s) that is resistive tohydrofluoric acid (HF) exposure such as, for example, silicon nitride(SiN) or silicon oxynitride (SiON). Formation of the protective guardlayer 116 may be implemented by a highly conformal process, such as bylow pressure chemical vapor deposition (LPCVD) for example.

Referring next to FIG. 5, a directional etch of the protective guardlayer 116 is employed to a point at which portions of the guard layer116 are initially removed from horizontal surfaces. Then, as shown inFIG. 6, the directional etch is continued until vertical portions of theguard layer 116 on the exposed fin structures 106 are removed, leavingvertical portions of the guard layer 116 only on lower sidewall portionsof the trench 114, and thereby defining a trench guard ring 116′. Theheight of the guard ring 116′ is sufficient to protect lower sidewallportions of the trench dielectric 112 from HF erosion, therebypreventing undesired lower trench widening that could result insubsequent epitaxial semiconductor material from being deposited inareas that are not desired.

FIG. 7 illustrates an HF precleaning operation of the structure of FIG.6 in preparation for epitaxial semiconductor material formation,resulting in the erosion of upper sidewall portions of the trench 114where the guard layer 116 was previously removed. Such recessed(widened) regions of the upper portion of the trench are indicated at118 in FIG. 7. Then, as shown in FIG. 8, a semiconductor epitaxialgrowth process is performed to result in merging of the exposed finstructures 106. That is, the formed epitaxial semiconductor material 120within the trench 114 extends at least above a height of thesemiconductor fins 106 so as to merge the semiconductor fins. The addedepitaxial semiconductor material 120 be the same as, or different fromthe fin semiconductor material. For example, the epitaxial semiconductormaterial may be, for example, single crystalline silicon, singlecrystalline germanium, a single crystalline alloy of at least two ofsilicon, germanium, and carbon, a single crystalline compoundsemiconductor material, a polycrystalline elemental semiconductormaterial, a polycrystalline alloy of at least two of silicon, germanium,and carbon, a polycrystalline compound semiconductor material, or anamorphous semiconductor material. In one embodiment, the epitaxialsemiconductor material is single crystalline.

In contrast to relatively thin epitaxial processes (e.g., for extensionformation), the trench epitaxial deposition is a relatively thickprocess that is not self-aligned to semiconductor surfaces. Thedeposition may be implemented, for example, by flowing a reactant gasincluding a precursor for the semiconductor material in a processingchamber in which the semiconductor structure is placed for processing.Exemplary precursors to the semiconductor material include, but are notlimited to, SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, GeH₄, Ge₂H₆, GeH₂Cl₂,and other precursor materials for compound semiconductor materials asknown in the art.

It is again pointed out that as a result of the trench guard ring 116′,deposition of the semiconductor material 120 is confined to thepatterned trench region originally defined in FIG. 3 and not expanded asthe result of an HF attack on the ILD material 112. Then, as shown inFIG. 9 and the top view of FIG. 10, a silicide layer 122 is formed overthe epitaxial semiconductor material 120. The silicide layer 122 isformed by deposition of a refractory metal such as, for example, nickel(Ni), platinum (Pt), cobalt (Co), titanium (Ti), tantalum (Ta), andcobalt (Co), over the entire structure. A reaction is induced during ananneal at an elevated temperature between the metal of the temporarymetal layer and the underlying semiconductor material 120 to form ametal semiconductor alloy portions (i.e., the silicide layer 122). Thetemperature of the anneal can be, for example, between 300° C. and 1000°C. Unreacted portions of the temporary metal layer are removed selectiveto the silicide layer 122, for example, by a wet etch. From this point,processing may continue as known in the art.

While the invention has been described with reference to an exemplaryembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

The invention claimed is:
 1. A method of forming a semiconductor device,the method comprising: forming an insulator layer over a substrate;opening a trench in the insulator layer so as to expose one or moresemiconductor structures formed on the substrate; forming a protectivelayer on sidewalls of the trench; subjecting the substrate to aprecleaning operation in preparation for epitaxial semiconductorformation, wherein the protective layer prevents expansion of thesidewalls of the trench as a result of the precleaning operation; andforming epitaxial semiconductor material within the trench and over theexposed one or more semiconductor structures; wherein forming theprotective layer on sidewalls of the trench further comprises: formingthe protective layer over the insulator layer and the exposed one ormore semiconductor structures; and anisotropically etching theprotective layer so as to remove horizontal portions thereof, and toremove vertical portions thereof from sidewall surfaces of the one ormore semiconductor structures, wherein anisotropically etching theprotective layer further results in removal of vertical portions of theprotective layer at an upper region of the sidewalls of the trench whilemaintaining vertical portions of the protective layer at a lower regionof the sidewalls of the trench.
 2. The method of claim 1, wherein theinsulator layer comprises silicon dioxide, and the precleaning operationincludes exposing the substrate to hydrofluoric acid.
 3. The method ofclaim 2, wherein the protective layer comprises one or more of siliconnitride and silicon oxynitride.
 4. The method of claim 1, wherein theone or more semiconductor structures comprise semiconductor fins.
 5. Themethod of claim 4, wherein the formed epitaxial semiconductor materialwithin the trench extends above a height of the semiconductor fins so asto merge the semiconductor fins.
 6. A method of forming a semiconductordevice, the method comprising: forming a plurality of semiconductor finsover a substrate; forming an insulator layer over the semiconductor finsand the substrate; opening a trench in the insulator layer so as toexpose a selected region of the plurality of semiconductor fins; forminga protective layer on sidewalls of the trench; subjecting the substrateto a precleaning operation in preparation for epitaxial semiconductorformation, wherein the protective layer prevents expansion of thesidewalls of the trench as a result of the precleaning operation; andforming epitaxial semiconductor material within the trench and over theexposed plurality of semiconductor fins so as to merge the semiconductorfins; wherein forming the protective layer on sidewalls of the trenchfurther comprises: depositing the protective layer over the insulatorlayer and the exposed one or plurality of semiconductor fins; andanisotropically etching the protective layer so as to remove horizontalportions thereof, and to remove vertical portions thereof from sidewallsurfaces of the plurality of semiconductor fins, wherein anisotropicallyetching the protective layer further results in removal of verticalportions of the protective layer at an upper region of the sidewalls ofthe trench while maintaining vertical portions of the protective layerat a lower region of the sidewalls of the trench.
 7. The method of claim6, wherein the precleaning operation results in widening of the upperregion of the sidewalls of the trench while the lower region of thesidewalls of the trench remains substantially intact.
 8. The method ofclaim 6, wherein the insulator layer comprises silicon dioxide, and theprecleaning operation includes exposing the substrate to hydrofluoricacid.
 9. The method of claim 8, wherein the protective layer comprisesone or more of silicon nitride and silicon oxynitride.
 10. The method ofclaim 6, further comprising forming a silicide layer over the epitaxialsemiconductor material and plurality of semiconductor fins.